The events in G1 which we propose to study are the replication and ciliation of the centriole in the centrosome. We will use anti-tubulin antibody and indirect immunofluorescence to visualize the centrosome containing the centriole and associated structures. The ultrastructure and function of the centrioles in non-neoplastic cells arrested in Gl (G8) by restrictive growth conditions (high cell density, low serum, poor substratum) will be compared to the same cells when restrictive conditions have been removed, and in exponentially growing cells. Relevant characteristics of the centriole's changes during G1 will be further analyzed at the ultrastructural level by electron microscopy. Once changes in the centriole's function which are associated with growth regulatory events in G1 are identified, then drugs or irradiation which inhibit centriole function will be used to examine the possible causal connection between centriole activities and the initiation of DNA synthesis. Differences in such events will then be sought between neoplastic and non-neoplastic cells under comparable conditions.